Hydraulic float control systems, in which floating positions have been implemented that permit a free movement of a hydraulic consumer, are known in the state of the art. Here, both connecting sides of the hydraulic consumer are connected to each other as well as at low pressure or without pressure to a hydraulic container or reservoir. Float control systems of this type are applied to construction or loader vehicles in which a boom or loader arm can be raised or lowered by means of a lifting cylinder. The function of the floating position is required, for example, to make it possible for a tool attached to the boom or loader arm to follow the contour of the ground closely independently of the position or orientation of the vehicle. Thereby, the tool is forced against the ground solely by the force of gravity. Such control systems do not contain any load-holding valves that prevent or sharply slow an unintended lowering of the boom or loader arm for reasons of safety when a leakage is encountered in the connection between the cylinder and control valve. Since a control pressure is required for the opening or circumventing of the load holding valve, a solution of the combination of a load-holding valve with a floating position is not known, in this case a non-pressurized condition of the hydraulic consumer is present and hence no control pressure can be used.
DE 101 49 787 A1 discloses a control system including a floating condition to control a double-acting consumer in which a control valve is loaded with pressure in a through-flow position and a floating position can be attained by a pressure controlled valve arrangement. Throttling is brought about over the control edges of the control valve where changes in the velocity of the consumer upon the transition into the floating position should effectively be avoided. Here the disadvantage is that the throttling is performed over a control arrangement supplied by a pump that is costly in its configuration as a control valve and results in a high degree of control inertia and at high loads unintended and uncontrolled pivoting movements result upon shifting to a floating position despite the throttling. Furthermore, the control system does not contain a load-holding valve to secure the hydraulic operation of the consumer.
DE 100 06 908 A1 discloses a hydraulic piston-cylinder unit for agricultural utility machines with a load-holding valve, in which an operating position is reached in which a constant pressure can be adjusted in the piston side chamber of the cylinder. Thereby a boom or a tool attached to it can be brought into contact with the ground constantly with a pre-determined contact force. This operating position is reached in that the pressure chambers of the piston-cylinder unit are connected to each other and pressure equalization occurs between the two pressure chambers over a pressure control valve. If the pressure drops below a pre-determined value, the pressure control valve closes. Here a floating position is possible only if the pre-determined value is set to zero, so that no pressure control is performed. The disadvantage then is that upon switching under load, the boom or the tool would be lowered uncontrolled.
DD 205 471 discloses a hydraulic circuit arrangement in which a floating position can be established upon the desire of an operator in which the chambers of a cylinder are connected with a container or sump by means of a three-position, two-way valve. A throttled outflow is guaranteed on the pressure side of the cylinder over a one-way restrictor in the operating position as well as in the floating position. The disadvantage is that upon switching into the floating position under load, the constant cross section of the throttle, designed for the operation, cannot control the lowering of the pressure side. Furthermore such a one-way restrictor does not represent a load-holding valve, which could prevent an uncontrolled lowering of the load in its operating position.